There has been growing interest in the potential of electrolytic reduction processes (also sometimes referred to as electro-decomposition and electro-deoxidation see for example U.S. Pat. No. 8,992,758, the disclosure of which is hereby incorporated by reference) as a means of producing a range of metals, and alloys of those metals, from feedstocks comprising compounds, particularly oxides, of the metals. Such a process, commonly known as the FFC Cambridge process, is disclosed in international patent publication WO99/64638 (the disclosure of which is hereby incorporated by reference), which broadly describes a method for removing a substance X from a solid metal or semi-metal compound M1X via electrolysis in a melt of M2Y. The substance X may be dissolved within M1 or the compound M1X may be a surface coating on a body of M1. The electrolysis is conducted under conditions such that reaction and therefore extraction of X rather than M2 deposition occurs at an electrode surface (where typically the electrode is formed from the M1X material), and that X dissolves in the electrolyte M2Y. The process is typically conducted at an elevated temperature, e.g. in the range 700° C.-1,000° C., above the melting point of M2Y but below its substantially higher boiling point. The elevated temperature is required to ensure an adequate rate of ionisation and diffusion of the substance X from the surface of the M1X.
In practical terms, the substance X is oxygen and the process has been of particular interest for the purpose of producing titanium metal product. It is known that where a mixture of oxides are reduced by the process, an alloy of the reduced metals will form, and further known that the configuration of the oxides in the feedstock will be largely maintained in the metal alloy end product. The development of a range of titanium alloys has focused primarily around the manipulation of the phase structure of the metal to produce the desired properties (for example—strength, ductility, modulus, fatigue and corrosion) by alloying with elements that stabilise the alpha and beta phases. Alpha phase stabilisers are Al, Ge, Ga, Sn and Zr. Aluminium is particularly favoured due to cost. Beta phase stabilisers are predominantly transition elements and include Mo, V, Ta, Nb, Mn, Fe, Cr, W, Co, Ni, Cu and Si.
There has therefore been interest in developing titaniferous feedstocks for electrolytic reduction processes that contain alloying elements tailored to produce a desired end product alloy. Known primary feeds to the process for the purpose of producing titanium alloys include natural rutile and synthetic rutile (see for example WO2013/050772A3, the disclosure of which is hereby incorporated by reference) as well as the pure product from the TiO2 pigment process, and efforts have been made to add the alloying elements discretely (also sometimes referred to doping) to the feed to the electrolysis cell. The alternative of adding alloying elements upstream of the electrolysis, while achieving diffusion and homogeneity in the grains, requires a costly high temperature treatment step.
Another possibility is to mix titanium metal powder with metal powders of the alloying elements downstream. However, this would require significant post processing to ensure homogeneity for developing the alloy properties, and would therefore involve similar issues to the adding of alloy powders to the rutile or synthetic rutile upstream of the electrolysis cell.
It is therefore an object of the invention to provide an economically attractive means of producing titanium alloys by an electrolytic reduction process (also known as electro-decomposition and electro-deoxidation processes).
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.